In a long term evolution (LTE) system, a user equipment (UE) may perform a random access procedure to obtain uplink resources required for uplink synchronization and/or communication. When the UE performs random access, the UE sends a random access preamble (RAP) to an eNB. Depending on whether the RAP is UE-specific, the random access procedure breaks down into a non-contention based random access procedure and a contention-based random access procedure. In the non-contention based random access, a dedicated RAP allocated by an eNB to the UE is applied.
In at least the following scenarios, a non-contention based random access procedure may be initiated. One scenario is that downlink data arrives at the eNB side but the UE is out of synchronization in the uplink at this time; and another scenario is that the eNB side instructs the UE to perform cell handover. In a non-contention based random access procedure, the UE sends a dedicated RAP to the eNB; and, if the UE receives a random access response (RAR) sent by the eNB, the UE determines success of the random access. If the UE receives no RAR in a specified time window, the RAP may be sent again immediately or after a period to trigger the whole random access procedure; and such actions are repeated until the random access of the UE succeeds or until a preset maximum random access count is reached which triggers an action other than the random access.
In the prior art, the RAR schedules transmission by using physical downlink control channel (PDCCH) control signaling that is sent in a common search space (CSS) and masked by using a random access radio network temporary identifier (RA-RNTI). Each RA-RNTI corresponds to a PRACH resource. In an LTE system, one PRACH resource is a length that can accommodate a RAP in a time domain, and is a size of six physical resource blocks in a frequency domain. Each PRACH resource can bear multiple RAPs. For example, in a possible implementation manner, an RA-RNTI corresponds to 64 RAPs. Different UEs that send different RAPs on the same PRACH resource use the same RA-RNTI to receive the RAR. Therefore, although different UEs use respective dedicated RAP when initiating a non-random access procedure, the dedicated RAPs may correspond to the same RA-RNTI. In addition, the RA-RNTI is independent of each carrier. That is, the RA-RNTIs used on different carriers may be the same, for example, the RA-RNTI corresponding to the RAP, which is sent by UE 1 on carrier 1, may be the same as the RA-RNTI corresponding to the RAP, which is sent by UE 2 on carrier 2.
For the RAPs sent by multiple UEs on the same carrier, if the RAPs correspond to the same RA-RNTI, multiple RARs in response to the RAPs of the multiple UEs may be set into a medium access control (MAC) protocol data unit (PDU). Therefore, in the same PDCCH transmission scheduling process, the eNB uses an RA-RNTI to mask PDCCH control signaling to schedule transmission of an RAR. The UE uses the RA-RNTI to perform blind detection for the PDCCH control signaling, and receives the RAR from the PDSCH indicated by the control signaling. If the correctly received RAR carries an identifier compliant with the UE-specific RAP, it indicates that the RAR is an RAR sent to the UE. Otherwise, it indicates that the RAR is not an RAR sent to the UE. Before an RAR receiving timer times out, the UE continues the blind detection of the control signaling masked by using the RA-RNTI and the corresponding RAR until the RAR sent to the UE is received successfully. If the RAR receiving timer times out and the UE has not successfully received the RAR sent to the UE, the UE sends a dedicated RAP to the eNB again, and the foregoing procedure is repeated. Therefore, it can be seen that the complexity for a UE to receive a random access response is high in the prior art.